1. Field of the Invention
The present invention relates to sensor systems and methods for detecting superstrates on or near the sensor and, more specifically, to a sensor system including transmission line sensors and methods for detecting and identifying superstrates such as, for example, coatings of ice on an airplane wing or road.
2. Description of the Prior Art
Identification of the presence, absence, and type of coating or superstrate on a suitably shaped sensor can be extremely useful. For instance, it would be highly desirable to detect the presence of ice on airplane wings, bridges, and roads with a sensor that conforms to the shape of the surface to be measured. Other applications for such a sensor include, for instance, detection of surface buildup in pipelines, detection of thin coatings, i.e., paints, oils, and the like, and proximity detection for automated machinery or robots.
Airlines have expressed a special interest in an ice detection system that meets certain requirements such as the ability to distinguish between ice and other contaminants such as antifreeze that may be on the wing. While identifying the presence or absence of ice is a major objective, one aspect of such a system should preferably include means to give accurate reading on the thickness and rate of ice buildup, if ice is present. The sensor should not influence the aerodynamics of the surface to be protected. The system should be compact. The system should be of sturdy construction, preferably with few components, and contain no parts that could work loose in service. Preferably the sensor should have a metallic surface so that ice adheres to the sensor in the same manner that it adheres to metallic wing surfaces so as to provide accurate readings.
Measuring ice buildup on airplanes is prompted by an increased concern over recent airplane crashes which were blamed on wing icing. Actual crashes are not the only concern. Each year airlines use about 10 million gallons of toxic ethylene glycol, entailing millions of dollars in material and cleanup cost. Many delays at airports result due to the time consuming de-icing process. These problems could be greatly reduced if a system were available to notify the pilot, if indeed, there is ice building up on the wing.
With respect to ice on roads and bridges, the highway departments spends millions of tax dollars each year for assuring that roadways are clear of ice and snow. Many tons of sand and salt are spread on roads that have not and will not accumulate ice. Moreover because the specific locations of iced areas are not known, the logistics and time required to spread sand and salt on all roads increases the time before the actually ice endangered roads are worked on. The introduction of an ice detector to the roadways, especially on bridges and other overpasses, could greatly reduce this waste as well as improve safety and time. The cost of the system would be quickly returned in savings. This type of application is very similar to the implementation of the sensor on wings of airplanes. It too would be required to be flush to the road and have the ability to give an accurate reading of the presence of ice buildup on the road.
As another example, Oil Companies have had a problem for many years now with superstrate buildup. As oil flows through a pipe, over time a solid residue begins to form on the inside of the pipe causing the flow of oil to become much less efficient. Eventually, the Oil Company must flush out this residue by sending a chemical through the line that liquefies the substance and returns the flow to normal. The process is quite costly. In an attempt to minimize the frequency of this process, oil companies have expressed a desire to know when a significant amount of residue has accumulated. The same type of needs may be found in refineries or other pipeline fluids.
For ice detectors, there are currently many methods being proposed for ice detection on airplane wings, including antennas, piezoelectric transducers, ultrasonic transducers, optical occlusion, and airflow sensors. With respect to sensors useful for operation in detecting ice on an airplane wing, the prior art sensors have one or more deficiencies. They may have a low sensitivity to thin layers of ice or do not conform to an airplane wing. The cost, complexity, and/or size may prohibit such use. They may not have the ability to distinguish between a variety of superstrates. Finally, the reliability may not be sufficient especially under the widely variable conditions of operation.
Some devices may measure thickness once the type of material is known. For instance, a microwave ice accretion measurement device instrument (MIAMI) developed by Ideal Research, Inc. under NASA Lewis sponsorship consists of a dielectric waveguide whose resonant frequency changes with superstrate dielectric and thickness. However, the MIAMI device does not have a metallic surface that is similar to the surface of airplane wings. This type of device or other type of device for detecting thickness of a known superstrate could be used in conjunction with one embodiment of the present invention that detects ice layers as thin as one millimeter.
The following patents disclose attempts to solve the above-discussed problems and related problems.
U.S. Pat. No. 5,551,288, issued Sep. 3, 1996 to Geraldi et al., discloses an improved ice sensor which is particularly effective in measuring and quantifying non-uniform, heterogeneous ice typically found on aircraft leading edges and top wing surfaces. In one embodiment, the ice sensor comprises a plurality of surface mounted capacitive sensors, each having a different electrode spacing. These sensors measure ice thickness by measuring the changes in capacitance of the flush electrode elements due to the presence of ice or water. Electronic guarding techniques are employed to minimize baseline and parasitic capacitances so as to decrease the noise level and thus increase the signal to noise ratio. Importantly, the use of guard electrodes for selective capacitive sensors also enables distributed capacitive measurements to be made over large or complex areas, independent of temperature or location, due to the capability of manipulating the electric field lines associated with the capacitive sensors.
U.S. Pat. No. 5,569,850, issued Oct. 29, 1996 to Richard L. Rauckhorst, III, discloses an ice detector which includes a pair of electrodes connected by a pair of leads to a control unit which measures the impedance (or other parameters) between leads to thereby sense and detect ice or other contaminants formed on top thereof. Electrodes are integrated into a patch and comprised of a top layer of conductive resin, a middle layer of conductive cloth and a bottom layer of conductive resin.
U.S. Pat. No. 5,474,261, issued Dec. 12, 1995 to Stolarczyk et al., discloses an ice detection system that comprises a network of thin, flexible microstrip antennas distributed on an aircraft wing at critical points and multiplexed into a microcomputer. Each sensor antenna and associated electronics measures the unique electrical properties of compounds that accumulate on the wing surface over the sensor. The electronics include provisions for sensor fusion wherein thermocouple and acoustic data values are measured. A microcomputer processes the information and can discern the presence of ice, water frost, ethylene-glycol or slush. A program executing in the microcomputer can recognize each compound's characteristic signal and can calculate the compounds thicknesses and can predict how quickly the substance is progressing toward icing conditions. A flight deck readout enables a pilot or ground crew to be informed as to whether de-icing procedures are necessary and/or how soon de-icing may be necessary.
U.S. Pat. No. 5,781,115, issued Jul. 14, 1998 to Donald F. Shea, discloses a system and method for detecting materials on a conductive surface and measuring the thickness and permittivity of the material. A polarized Radio Frequency signal is reflected from a conduction surface having a material thereon. The reflected de-polarized signal is then processed to determine the thickness and permittivity of the material on the conductive surface.
U.S. Pat. No., 5,005,015, issued Apr. 2, 1991, to Dehn et al., discloses a system and method for detecting the state and thickness of water accumulation on a surface incorporating a plurality of spaced, thin, electrically resonant circuits bonded to the surface and a radio frequency transmitter for exciting the circuits to resonance. A receiver detects the resonant signal from each circuit, determines the resonant frequency and quality factor of the circuit and correlates that information with predetermined data representing changes in resonant frequency and quality factor as a function of liquid water and ice accretion to thereby establish the state and thickness of water overlaying the circuits.
U.S. Pat. No. 4,766,396, issued Aug. 23, 1988, to Taya, et al., discloses a current source type current output circuit for applying to a load a current which is proportional to an input includes an amplifier of the type receiving a current and producing a voltage, and a feedback circuit for feeding back an output of the amplifier to an input terminal of the amplifier. The feedback circuit is made up of a first, a second, and a third current mirror circuit, and a first, a second, and a third resistor. An output terminal of the amplifier is connected to an input terminal of the second current mirror circuit via the third resistor and to an input terminal of the first current mirror circuit via a series connection of the first and second resistors. The load is connected to the intermediate point of the serial connection of the first and second resistors. An output terminal of the second current mirror circuit is connected to an input terminal of the third current mirror circuit. Output terminals of the first and third current mirror circuits are connected to an input terminal of the amplifier such that a current which is proportional to an input is fed to the load. A reference terminal of each of the first and second current mirror circuits is connected to a first power source, and a reference terminal of the third current mirror circuit is connected to a second power source.
U.S. Pat. No. 4,688,185, issued Aug. 18, 1987 to Magenheim et al., discloses an ice measurement instrument including a waveguide operating in a transmission mode passing energy from an input port to an output port. The resonant frequency of the waveguide depends on the presence and/or thickness of ice at a measuring location. The energy applied to the input port is swept in frequency from a first frequency to a second frequency at or above an ice-free resonant frequency of said waveguide, and back to said first frequency. Energy received at the output port is peak detected to provide a detection signal with four recognizable transitions identifying a pair of peaks which correspond to the resonant frequency of the waveguide. The time delay between these peaks can be used, in comparison with the time delay corresponding to an ice-free condition, to determine ice thickness.
U.S. Pat. No. 4,649,713, issued Mar. 17, 1987, to Donald J. Bezek, discloses a sensing and control device provided for monitoring the build up of frost, ice and condensate on the cooling coils of refrigeration unit. The microwave unit is placed a fixed distance away from a cooling coil and provides an emitted wave and reflected wave. The reflected wave, and the resulting standing wave, shift in spacial phase which differs due to the accumulation of ice or frost and provides a voltage change which is observed by an electronic circuit to shut off until the ice melts. The sensing and control unit is also used to sense the removal of ice and frost by heating of the defrost cycle and thus establish the termination of defrost cycle and restoration of refrigeration. The microwave sensing device permits the refrigeration unit to be cycled on and off to prevent an excessive build-up of ice which would dramatically lower unit efficiency by preventing the circulation of cooling air across the heat exchanger or coil as it is called to circulate cool air into the contiguous space.
U.S. Pat. No. 4,470,123, issued on Sep. 4, 1984, to Magenheim et al., discloses a system for indicating ice thickness and rate of ice thickness growth on surfaces. The region to be monitored for ice accretion is provided with a resonant surface waveguide which is mounted flush, below the surface being monitored. A controlled oscillator provides microwave energy via a feed point at a controllable frequency. A detector is coupled to the surface waveguide and is responsive to electrical energy. A measuring device indicates the frequency deviation of the controlled oscillator from a quiescent frequency. A control means is provided to control the frequency of oscillation of the controlled oscillator. In a first, open-loop embodiment, the control means is a shaft operated by an operator. In a second, closed-loop embodiment, the control means is a processor which effects automatic control.
U.S. Pat. No. 4,054,255, issued Oct. 18, 1977, to Bertram Magenheim, discloses a system for detecting ice on exterior surfaces of aircraft by transmitting a relatively low power microwave electromagnetic signal into a dielectric layer functioning as a surface waveguide, and monitoring the signals transmitted into and reflected from the waveguide. The waveguide includes a termination element which is mismatched with the waveguide impedance, resulting in partial or total reflection of the microwave energy from the remote end of the waveguide. As ice builds up on the surface waveguide, the impedance or reflection characteristics of the composite waveguide comprising the ice layer and the permanent surface waveguide give a reliable indication of the presence and location of the ice. The reflection characteristics are conventionally monitored utilizing a dual directional coupler and a reflectometer.
U.S. Pat. No. 5,497,100, issued Mar. 5, 1996, to Reiser et al., discloses a surface condition sensing system which includes a frequency controlled source of electromagnetic power adapted to produce a band of selected frequencies which are directed to a surface under examination. A monitoring circuit compares transmitted and reflected electromagnetic power as a function of frequency from the surface, and generates a plurality of absorption signals representing the difference between the amplitude of the transmitted signal with the corresponding amplitude of the reflected signal. An evaluator circuit generates a surface condition signal representing the results of a comparison between the plurality of absorption signals with known surface models. A control circuit generates a status signal representative of the surface condition.
U.S. Pat. No. 5,772,153, issued Jun. 30, 1998, to Abaunza et al., discloses an icing sensor utilizing a surface gap transmission line along which a radio frequency is transmitted. The phase delay of the radio frequency along the transmission line is dependent upon the dielectric constant presented at the surface in the gap between the transmission line electrodes. Accordingly, changes of dielectric constant affect phase delay of the transmitted frequency. This phase delay may be used to detect the difference between ice, water and snow as well as the presence of freezing point depressing fluids such as ethylene glycol. When the sensor is mounted on an aircraft control surface, the presence and likelihood of icing conditions may be predicted. Through the use of one or more temperature, freezing point depressing fluids/water mixture determined from dielectric constant, and rate of change of the dielectric constant, it is possible to predict the time delay until icing begins. Thus, the sensor of the present application may safely reduce the effort and expense in aircraft de-icing.
The above cited prior art does not provide a sensor that is conformable to a surface and extendable along the length of a surface, such as an airplane wing, that provides information about the type of material of superstrate on the sensor and the location of an ice superstrate along the sensor. The sensor(s) of the present invention may be used on conductive and non-conductive surfaces. Multiple sensors may be used with one quadrature phase detector. The prior art does not disclose sensors that are spaced along a transmission line to provide additive phase shift at the detector making it possible to have ten or more sensors on one strip or transmission line covering many feet of surface. Moreover, the prior art does not disclose sensors that can be spaced at desired intervals by changing the frequency of operation as well as by spacing along the transmission line. The cited art does not provide for a sensor as described that detects very thin coatings of a superstrate such that it is sensitive to a one millimeter coating of a superstrate such as ice. The prior art does not include a computer model operable to test various sensor configurations and provide additional baseline data.
Consequently, there is a strong need for such a sensor that would be useful in many applications such as detecting ice on an airplane wing. Those skilled in the art have long sought and will appreciate the present invention that addresses these and other problems.